Fiber communication systems and methods

ABSTRACT

An injection locked transmitter for an optical communication network includes a primary seed laser source input substantially confined to a single longitudinal mode, an input data stream, and a laser injected modulator including at least one secondary laser having a resonator frequency that is injection locked to a frequency of the single longitudinal mode of the primary seed laser source. The laser injected modulator is configured to receive the primary seed laser source input and the input data stream, and output a laser modulated data stream.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/600,324, filed on Oct. 11, 2019. U.S. application Ser. No. 16/600,324is a continuation of U.S. application Ser. No. 15/861,303, filed on Jan.3, 2018, now U.S. Pat. No. 10,447,404. U.S. application Ser. No.15/861,303 is a continuation of U.S. application Ser. No. 15/283,632,filed on Oct. 3, 2016, now U.S. Pat. No. 9,912,409. U.S. applicationSer. No. 15/283,632 claims the benefit of and priority to U.S.Provisional Patent Application Ser. No. 62/321,211, filed Apr. 12, 2016.All of these prior applications are incorporated herein by reference intheir entireties.

BACKGROUND

The field of the disclosure relates generally to optical communicationnetworks, and more particularly, to optical networks utilizingwavelength division multiplexing.

Telecommunications networks include an access network through which enduser subscribers connect to a service provider. Bandwidth requirementsfor delivering high-speed data and video services through the accessnetwork are rapidly increasing to meet growing consumer demands. Atpresent, data delivery over the access network is growing by gigabits(Gb)/second for residential subscribers, and by multi-Gb/s for businesssubscribers. Present access networks are based on passive opticalnetwork (PON) access technologies, which have become the dominant systemarchitecture to meet the growing high capacity demand from end users.

Gigabit PON and Ethernet PON architectures are conventionally known, andpresently provide about 2.5 Gb/s data rates for downstream transmissionand 1.25 Gb/s for upstream transmission (half of the downstream rate).10 Gb/s PON (XG-PON or IEEE 10G-EPON) has begun to be implemented forhigh-bandwidth applications, and a 40 Gb/s PON scheme, which is based ontime and wavelength division multiplexing (TWDM and WDM) has recentlybeen standardized. A growing need therefore exists to develophigher/faster data rates per-subscriber to meet future bandwidth demand,and also increase the coverage for services and applications, but whilealso minimizing the capital and operational expenditures necessary todeliver higher capacity and performance access networks.

One known solution to increase the capacity of a PON is the use of WDMtechnology to send a dedicated wavelength signal to end users. Currentdetection scheme WDM technology, however, is limited by its low receiversensitivity, and also by the few options available to upgrade and scalethe technology, particularly with regard to use in conjunction with thelower-quality legacy fiber environment. The legacy fiber environmentrequires operators to squeeze more capacity out of the existing fiberinfrastructure to avoid costs associated with having to retrench newfiber installment. Conventional access networks typically include sixfibers per node, servicing as many as 500 end users, such as homesubscribers. Conventional nodes cannot be split further and do nottypically contain spare (unused) fibers, and thus there is a need toutilize the limited fiber availability in a more efficient andcost-effective manner.

Coherent technology has been proposed as one solution to increase bothreceiver sensitivity and overall capacity for WDM-PON optical accessnetworks, in both brown and green field deployments. Coherent technologyoffers superior receiver sensitivity and extended power budget, and highfrequency selectivity that provides closely-spaced dense or ultra-denseWDM without the need for narrow band optical filters. Moreover, amulti-dimensional recovered signal experienced by coherent technologyprovides additional benefits to compensate for linear transmissionimpairments such as chromatic dispersion (CD) and polarization-modedispersion (PMD), and to efficiently utilize spectral resources tobenefit future network upgrades through the use of multi-level advancedmodulation formats. Long distance transmission using coherenttechnology, however, requires elaborate post-processing, includingsignal equalizations and carrier recovery, to adjust for impairmentsexperienced along the transmission pathway, thereby presentingsignificant challenges by significantly increasing system complexity.

Coherent technology in longhaul optical systems typically requiressignificant use of high quality discrete photonic and electroniccomponents, such as digital-to-analog converters (DAC),analog-to-digital converters (ADC), and digital signal processing (DSP)circuitry such as an application-specific integrated circuit (ASIC)utilizing CMOS technology, to compensate for noise, frequency drift, andother factors affecting the transmitted channel signals over the longdistance optical transmission. Coherent pluggable modules for metrosolution have gone through C Form-factor pluggable (CFP) to CFP2 andfuture CFP4 via multi-source agreement (MSA) standardization to reducetheir footprint, to lower costs, and also to lower power dissipation.However, these modules still require significant engineering complexity,expense, size, and power to operate, and therefore have not beenefficient or practical to implement in access applications.

BRIEF SUMMARY

In one aspect, an injection locked transmitter for an opticalcommunication network includes a master seed laser source inputsubstantially confined to a single longitudinal mode, an input datastream, and a laser injected modulator including at least one slavelaser having a resonator frequency that is injection locked to afrequency of the single longitudinal mode of the master seed lasersource. The laser injected modulator is configured to receive the masterseed laser source input and the input data stream, and output a lasermodulated data stream.

In another aspect, an optical network communication system includes, aninput signal source, an optical frequency comb generator configured toreceive the input signal source and output a plurality of phasesynchronized coherent tone pairs. Each of the plurality of phasesynchronized coherent tone pairs includes a first unmodulated signal anda second unmodulated signal. The system further include a firsttransmitter configured to receive the first unmodulated signal of aselected one of the plurality of phase synchronized coherent tone pairsas a seed source and to output a first modulated data stream, and afirst receiver configured to receive the first modulated data streamfrom the first transmitter and receive the second unmodulated signal ofthe selected one of the plurality of phase synchronized coherent tonepairs as a local oscillator source.

In yet another aspect, an optical network communication system includesan optical hub including an optical frequency comb generator configuredto output at least one phase synchronized coherent tone pair having afirst unmodulated signal and a second unmodulated signal, and adownstream transmitter configured to receive the first unmodulatedsignal as a seed source and to output a downstream modulated datastream. The system further includes a fiber node and an end userincluding a downstream receiver configured to receive the downstreammodulated data stream from the downstream transmitter and receive thesecond unmodulated signal as a local oscillator source.

In a still further aspect, a method of optical network processingincludes steps of generating at least one pair of first and secondunmodulated phase synchronized coherent tones, transmitting the firstunmodulated phase synchronized coherent tone to a first transmitter as aseed signal, adhering downstream data, in the first transmitter, to thefirst unmodulated phase synchronized coherent tone to generate a firstmodulated data stream signal, optically multiplexing the first modulateddata stream signal and the second unmodulated phase synchronizedcoherent tone together within a hub optical multiplexer, andcommunicating the multiplexed first modulated data stream signal and thesecond unmodulated phase synchronized coherent tone to a first receiver,by way of fiber optics, for downstream heterodyne detection.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic illustration of an exemplary fiber communicationsystem in accordance with an exemplary embodiment of the presentdisclosure.

FIG. 2 is a schematic illustration depicting an exemplary transmitterthat can be utilized with the fiber communication system depicted inFIG. 1.

FIG. 3 is a schematic illustration depicting an alternative transmitterthat can be utilized with the fiber communication system depicted inFIG. 1.

FIG. 4 is a schematic illustration depicting an alternative transmitterthat can be utilized with the fiber communication system depicted inFIG. 1.

FIG. 5 is a schematic illustration depicting an alternative transmitterthat can be utilized with the fiber communication system depicted inFIG. 1.

FIG. 6 is a schematic illustration depicting an exemplary upstreamconnection that can be utilized with the fiber communication systemdepicted in FIG. 1.

FIG. 7 is a schematic illustration depicting an exemplary processingarchitecture implemented with the fiber communication system depicted inFIG. 1.

FIG. 8 is a flow chart diagram of an exemplary downstream opticalnetwork process.

FIG. 9 is a flow chart diagram of an exemplary upstream optical networkprocess that can be implemented with the downstream process depicted inFIG. 8.

Unless otherwise indicated, the drawings provided herein are meant toillustrate features of embodiments of this disclosure. These featuresare believed to be applicable in a wide variety of systems including oneor more embodiments of this disclosure. As such, the drawings are notmeant to include all conventional features known by those of ordinaryskill in the art to be required for the practice of the embodimentsdisclosed herein.

DETAILED DESCRIPTION

In the following specification and the claims, reference will be made toa number of terms, which shall be defined to have the followingmeanings.

The singular forms “a,” “an,” and “the” include plural references unlessthe context clearly dictates otherwise.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where the event occurs and instances where it does not.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” “approximately,” and “substantially,” are notto be limited to the precise value specified. In at least someinstances, the approximating language may correspond to the precision ofan instrument for measuring the value. Here and throughout thespecification and claims, range limitations may be combined and/orinterchanged; such ranges are identified and include all the sub-rangescontained therein unless context or language indicates otherwise.

FIG. 1 is a schematic illustration of an exemplary fiber communicationsystem 100 in accordance with an exemplary embodiment of the presentdisclosure. System 100 includes an optical hub 102, a fiber node 104,and an end user 106. Optical hub 102 is, for example, a central office,a communications hub, or an optical line terminal (OLT). In theembodiment shown, fiber node 104 is illustrated for use with a passiveoptical network (PON). End user 106 is a downstream termination unit,which can represent, for example, a customer device, customer premises(e.g., an apartment building), a business user, or an optical networkunit (ONU). In an exemplary embodiment, system 100 utilizes a coherentDense Wavelength Division Multiplexing (DWDM) PON architecture.

Optical hub 102 communicates with fiber node 104 by way of downstreamfiber 108. Optionally, where upstream communication is desired alongsystem 100, optical hub 102 further connects with fiber node 104 by wayof upstream fiber 110. In operation, downstream fiber 108 and upstreamfiber 110 are typically 30 km or shorter. However, according to theembodiments presented herein, greater lengths are contemplated, such asbetween 100 km and 1000 km. In an exemplary embodiment, fiber node 104connects with end user 106 by way of fiber optics 112. Alternatively,fiber node 104 and end user 106 may be integrated as a single device,such as a virtualized cable modem termination system (vCMTS), which maybe located at a customer premises. Where fiber node 104 and end user 106are separate devices, fiber optics 112 typically spans a distance ofapproximately 5000 feet or less.

Optical hub 102 includes an optical frequency comb generator 114, whichis configured to receive a high quality source signal 116 from anexternal laser 118 and thereby generate multiple coherent tones 120(1),120(1′), . . . 120(N), 120(N′). Optical frequency comb generator 114utilizes, for example, a mode-locked laser, a gain-switched laser, orelectro-optic modulation, and is constructed such that multiple coherenttones 120 are generated as simultaneous low-linewidth wavelengthchannels of known and controllable spacing. This advantageous aspect ofthe upstream input signal into system 100 allows a simplifiedarchitecture throughout the entire downstream portion of system 100, asdescribed further below.

Generated coherent tones 120 are fed into an amplifier 122, and theamplified signal therefrom is input into a first hub opticaldemultiplexer 124. In an exemplary embodiment, amplifier 122 is anerbium-doped fiber amplifier (EDFA). Optical hub 102 further includes adownstream transmitter 126 and a hub optical multiplexer 128. In anembodiment, optical hub 102 optionally includes a hub optical splitter130, an upstream receiver 132, and a second hub optical demultiplexer134.

Downstream transmitter 126 includes a downstream optical circulator 136and a downstream modulator 138. In an exemplary embodiment, downstreammodulator 138 is an injection locked laser modulator. Upstream receiver132 includes an upstream integrated coherent receiver (ICR) 140, anupstream analog to digital converter (ADC) 142, and an upstream digitalsignal processor (DSP) 144. In the exemplary embodiment, fiber node 104includes a node optical demultiplexer 146. In an alternative embodiment,where upstream transmission is desired, fiber node 104 further includesa node optical multiplexer 148. In the exemplary embodiment, nodeoptical demultiplexer 146 and node optical multiplexer 148 are passivedevices.

End user 106 further includes a downstream receiver 150. In an exemplaryembodiment, downstream receiver 150 has a similar architecture toupstream receiver 132, and includes a downstream ICR 152, a downstreamADC 154, and a downstream DSP 156. For upstream transmission, end user106 optionally includes end user optical splitter 158, which may belocated within downstream receiver 150 or separately, and an upstreamtransmitter 160. In an exemplary embodiment, upstream transmitter 160has a similar architecture to downstream transmitter 126, and includesan upstream optical circulator 162, and an upstream modulator 164.

In operation, system 100 utilizes optical frequency comb generator 114and amplifier 122 convert the input high quality source signal 116 intomultiple coherent tones 120 (e.g., 32 tones, 64 tones, etc.), which arethen input to first hub optical demultiplexer 124. In an exemplaryembodiment, high quality source signal 116 is of sufficient amplitudeand a narrow bandwidth such that a selected longitudinal mode of signal116 is transmitted into optical frequency comb generator 114 withoutadjacent longitudinal modes, which are suppressed prior to processing bycomb generator 114. First hub optical demultiplexer 124 then outputs aplurality of phase synchronized coherent tone pairs 166(1), 166(2), . .. 166(N). That is, the generated coherent frequency tones 120 areamplified by amplifier 122 to enhance optical power, and thendemultiplexed into multiple separate individual phased synchronizedcoherent tone source pairs 166. For simplicity of discussion, thefollowing description pertains only to coherent tone pair 166(1)corresponding to the synchronized pair signal for the first channeloutput, which includes a first unmodulated signal 168 for Ch1 and asecond unmodulated signal 170 for Ch1′, and their routing through system100.

With source signal 116 of a high quality, narrow band, and substantiallywithin a single longitudinal mode, coherent tone pair 166(1), includingfirst unmodulated signal 168 (Ch1) and second unmodulated signal 170(Ch1′), is output as a high quality, narrowband signal, which thenserves as both a source of seed and local oscillator (LO) signals forboth downstream and upstream transmission and reception directions ofsystem 100. That is, by an exemplary configuration, the architecture ofoptical frequency comb generator 114 advantageously produces highquality continuous wave (CW) signals. Specifically, first unmodulatedsignal 168 (Ch1) may function as a downstream seed and upstream LOthroughout system 100, while second unmodulated signal 170 (Ch1′)concurrently may function as an upstream seed and downstream LO forsystem 100.

According to the exemplary embodiment, within optical hub 102, firstunmodulated signal 168 (Ch1) is divided by hub optical splitter 130 andis separately input to both downstream transmitter 126 and upstreamreceiver 132 as a “pure” signal, and i.e., substantially low amplitude,narrow bandwidth continuous wave does not include adhered data. Firstunmodulated signal 168 (Ch1) thus becomes a seed signal for downstreamtransmitter 126 and an LO signal for upstream receiver 132. In anexemplary embodiment, within downstream transmitter 126, firstunmodulated signal 168 (Ch1) passes through downstream opticalcirculator 136 into downstream modulator 138, in which one or more laserdiodes (not shown in FIG. 1, described below with respect to FIGS. 2-5)are excited, and adhere data (also not shown in FIG. 1, described belowwith respect to FIGS. 2-5) to the signal that then exits downstreamoptical circulator 136 as downstream modulated data stream 172 (Ch1).

In an exemplary embodiment, downstream optical circulator 136 is withindownstream transmitter 126. Alternatively, downstream optical circulator136 may be physically located separately from downstream transmitter126, or else within the confines of downstream modulator 138. Downstreammodulated data stream 172 (Ch1) is then combined in hub opticalmultiplexer 128 with the plurality of modulated/unmodulated data streampairs from other channels (not shown) and transmitted over downstreamfiber 108, to a node optical demultiplexer 174 in fiber node 104, whichthen separates the different channel stream pairs for transmission todifferent respective end users 106. At end user 106, because the datastream pair 170, 172 entering downstream receiver 150 is a phasesynchronized, digital signal processing at downstream DSP 156 is greatlysimplified, as described below with respect to FIG. 7.

Where upstream reception is optionally sought at optical hub 102, secondunmodulated signal 170 (Ch1′) is divided, within end user 106, by enduser optical splitter 158 and is separately input to both downstreamreceiver 150 and upstream transmitter 160 as a “pure” unmodulated signalfor Ch1′. In this alternative embodiment, second unmodulated signal 170(Ch1′) thus functions a seed signal for upstream transmitter 160 and a“pseudo LO signal” for downstream receiver 150 for the coherentdetection of Ch1. For purposes of this discussion, second unmodulatedsignal 170 (Ch1′) is referred to as a “pseudo LO signal” because it usesan LO signal from a remote source (output from first hub opticaldemultiplexer 124), and is not required to produce an LO signal locallyat end user 106. This particular configuration further significantlyreduces cost and complexity of the architecture of the system 100 by thereduction of necessary electronic components.

For upstream transmission, in an exemplary embodiment, a similarcoherent detection scheme is implemented for upstream transmitter 160 asis utilized for downstream transmitter 126. That is, second unmodulatedsignal 170 (Ch1′) is input to upstream optical circulator 162 andmodulated by upstream modulator 164 to adhere symmetric or asymmetricdata (not shown, described below with respect to FIG. 6) utilizing oneor more slave lasers (also not shown, described below with respect toFIG. 6), and then output as an upstream modulated data stream 176(Ch1′), which is then combined with similar modulated data streams fromother channels (not shown) by a node multiplexer 178 in fiber node 104.Second unmodulated signal 170 (Ch1′) is then transmitted upstream overupstream fiber 110, separated from other channel signals by second huboptical demultiplexer 134, an input to upstream receiver 132, forsimplified digital signal processing similar to the process describedabove with respect to downstream receiver 150.

By this exemplary configuration, multiple upstream channels fromdifferent end users 106 can be multiplexed at fiber node 104 (or aremote node) and sent back to optical hub 102. Thus, within optical hub102, the same coherent detection scheme may be used at upstream receiver132 as is used with downstream receiver 150, except that upstreamreceiver 132 utilizes first unmodulated signal 168 (Ch1) as the LO andupstream modulated data stream 176 (Ch1′) to carry data, whereasdownstream receiver 150 utilizes the data stream pair (Ch1, Ch1′) inreverse. That is, downstream receiver 150 utilizes second unmodulatedsignal 170 (Ch1′) as the LO and downstream modulated data stream 172(Ch1) to carry data.

Implementation of the embodiments described herein are useful formigrating hybrid fiber-coaxial (HFC) architectures towards other typesof fiber architectures, as well as deeper fiber architectures. TypicalHFC architectures tend to have very few fiber strands available fromfiber node to hub (e.g. fibers 108, 110), but many fiber strands couldbe deployed to cover the shorter distances that are typical from legacyHFC nodes to end users (e.g., fiber optics 112). In the exemplaryembodiments described herein, two fibers (i.e., fibers 108, 110) areillustrated between optical hub 102 and fiber node 104, which can be alegacy HFC fiber node. That is, one fiber (i.e., downstream fiber 108)is utilized for downstream signal and upstream seed/downstream LO, andanother fiber (i.e., upstream fiber 110) is utilized for upstreamsignal. Additionally, three fibers (i.e., fiber optics 112A-C) areillustrated for each end user from fiber node 104 (e.g., legacy HFCfiber node) to end user 106. By utilization of the advantageousconfigurations herein, fiber deeper or all-fiber migration schemes canutilize an HFC fiber node as an optical fiber distribution node, therebygreatly minimizing the need for fiber retrenching from an HFC node to anoptical hub.

The architecture described herein, by avoiding the need for conventionalcompensation hardware, can therefore be structured as a significantlyless expensive and more compact physical device than conventionaldevices. This novel and advantageous system and subsystem arrangementallows for multi-wavelength emission with simplicity, reliability, andlow cost. Implementation of optical frequency comb generator 114, withhigh quality input source signal 116, further allows simultaneouscontrol of multiple sources that are not realized by conventionaldiscrete lasers. According to the embodiments herein, channel spacing,for example, may be 25 GHz, 12.5 GHz, or 6.25 GHz, based on availablesignal bandwidth occupancy.

The embodiments described herein realize still further advantages byutilizing a comb generator (i.e., optical frequency comb generator 114)that maintains a constant wavelength spacing, thereby avoiding opticalbeat interference (OBI) that may be prevalent in cases with simultaneoustransmissions over a single fiber. In the exemplary embodimentillustrated in FIG. 1, fiber node 104 is shown as a passive system, andis thus expected to maintain a higher reliability than other migrationapproaches. Nevertheless, one of ordinary skill in the art, afterreading and comprehending present application, will understand how theembodiments disclosed herein may also be adapted to a remote PHYsolution, or to a remote cable modem termination system (CMTS) that isincluded in the fiber node.

As illustrated and described herein, system 100 may utilize anarchitecture of coherent DWDM-PON incorporate novel solutions to meetthe unique requirements of access environment, but with cost-efficientstructures not seen in conventional hardware systems. Optical frequencycomb generator 114 produces a plurality of simultaneous narrow widthwavelength channels with controlled spacing, thereby allowing simplifiedtuning of the entire wavelength comb. This centralized comb light sourcein optical hub 102 therefore provides master seeding sources and LOsignals for both downstream and upstream directions in heterodynedetection configurations in order to reuse the optical sourcesthroughout the entirety of system 100. This advantageous configurationrealizes significant cost savings and reduction in hardware complexityover intradyne detection schemes in long-haul systems, for example.

FIG. 2 is a schematic illustration depicting an exemplary downstreamtransmitter 200 that can be utilized with fiber communication system100, depicted in FIG. 1. Downstream transmitter 200 includes downstreamoptical circulator 136 (see FIG. 1, above) in two-way communication witha laser injected modulator 202, which includes a laser diode 204, whichreceives data 206 from an external data source 208. In an alternativeembodiment, downstream transmitter 200 may include two separate fiberreceivers (not shown), which would substitute, and eliminate the need,for downstream optical circulator 136 in the structural configurationshown.

In operation, downstream transmitter 200 performs the same generalfunctions as downstream transmitter 126 (FIG. 1, described above). Laserinjected modulator 202 utilizes laser diode 204 as a “slave laser.” Thatis, laser diode 204 is injection locked by external laser 118, whichfunctions as a single frequency or longitudinal mode master, or seed,laser to keep the frequency of a resonator mode of laser diode 204 closeenough to the frequency of the master laser (i.e., laser 118) to allowfor frequency locking. The principle of downstream transmitter 200 isalso referred to as “laser cloning,” where a single high quality masterlaser (i.e., laser 118) transmits a narrow bandwidth, low noise signal(i.e., source signal 116), and a relatively inexpensive slave laser(e.g., laser diode 204) can be used throughout system 100 to transmitdata modulated signals, such as downstream modulated data stream 172(Ch1). In an exemplary embodiment, laser diode 204 is a Fabry Perotlaser diode (FP LD), or a vertical-cavity surface-emitting laser(VCSEL), in comparison with the considerably more expensive distributedfeedback laser diodes (DFB LD) that are conventionally used. In analternative embodiment, laser diode 204 is an LED, which can perform asa sufficient slave laser source according to the embodiments herein dueto the utilization of the high quality source signal 116 that isconsistently utilized throughout system 100.

More specifically, first unmodulated signal 168 (Ch1) exiting huboptical splitter 130 is input to downstream optical circulator 136,which then excites laser diode 204, that is, laser diode 204 emits lightat a specified modulation rate. Laser injected modulator 202 adheresdata 206 to the excited Ch1 signal, and the resultant modulated Ch1signal with adhered data is output from downstream optical circulator136 as downstream modulated data stream 172 (Ch1). According to thisexemplary embodiment, first unmodulated signal 168 (Ch1) is input todownstream transmitter 126 as an unmodulated, low amplitude, narrowbandwidth, low noise “pure” source, and is modulated by laser diode 204,which is a high amplitude, wide bandwidth device, and resultantdownstream modulated data stream 172 (Ch1) is a high amplitude, narrowbandwidth, low noise “pure” signal that can be transmitted throughoutsystem 100 without the need for further conventional compensation means(hardware and programming). Suppression of adjacent longitudinal modesfrom laser diode 204, for example, is not necessary because of theexciting source signal (i.e., signal 168) is of such high quality andnarrow bandwidth that output downstream modulated data stream 172 (Ch1)is substantially amplified only within the narrow bandwidth of externallaser 118. In the exemplary embodiment illustrated in FIG. 2, laserinjected modulator 202 implements direct modulation.

Optical injection locking as described herein thus improves upon theperformance of the relatively less expensive, multi-longitudinal slavelaser source (i.e., laser diode 204) in terms of spectral bandwidth andnoise properties. With respect to heterodyne coherent detection,incoming signals (upstream or downstream) can be combined with the LO orpseudo-LO and brought to an intermediate frequency (IF) for electronicprocessing. According to this exemplary configuration, part of theLO/pseudo-LO optical power can also be employed as the master/seed laserfor the reverse transmission direction, at both optical hub 102, and atend user 106 (described below with respect to FIG. 6), and thus a fullycoherent system having a master seed and LO delivery from an optical hubcan be achieved in a relatively cost-effective manner comparison withconventional systems.

FIG. 3 is a schematic illustration depicting an alternative downstreamtransmitter 300 that can be utilized with fiber communication system100, depicted in FIG. 1. Downstream transmitter 300 is similar todownstream transmitter 200 (FIG. 2), including the implementation ofdirect modulation, except that downstream transmitter 300 alternativelyutilizes polarization division multiplexing to modulate the Ch1 signalinto downstream modulated data stream 172 (Ch1).

Downstream transmitter 300 includes downstream optical circulator 136(see FIG. 1, above) in two-way communication with a laser injectedmodulator 302, which includes a polarization beam splitter(PBS)/polarization beam combiner (PBC) 304, which can be a singledevice. Laser injected modulator 302 further includes a first laserdiode 306 configured to receive first data 308 from an external datasource (not shown in FIG. 3), and a second laser diode 310 configured toreceive second data 312 from the same, or different, external datasource.

In operation, downstream transmitter 300 is similar to downstreamtransmitter 200 with respect to the implementation of direct modulation,and master/slave laser injection locking. Downstream transmitter 300though, alternatively implements dual-polarization from the splitterportion of PBS/PBC 304, which splits first unmodulated signal 168 (Ch1)into its x-polarization component P1 and y-polarization component P2,which separately excite first laser diode 306 and second laser diode310, respectively. Similar to downstream transmitter 200 (FIG. 2), indownstream transmitter 300, first unmodulated signal 168 (Ch1) exitinghub optical splitter 130 is input to downstream optical circulator 136,the separate polarization components of which then excite laser diodes306, 310, respectively, at the specified modulation rate. Laser injectedmodulator 302 adheres data first and second data 308, 312 to therespective excited polarization components of the Ch1 signal, which arecombined by the combiner portion of PBS/PBC 304. The resultant modulatedCh1 signal with adhered data is output from downstream opticalcirculator 136 as downstream modulated data stream 172 (Ch1).

In an exemplary embodiment, the polarized light components received byfirst and second laser diodes 306, 310 are orthogonal (90 degrees and/ornoninteractive). That is, first laser diode 306 and second laser diode310 are optimized as slave lasers to lock onto the same wavelength asexternal laser 118 (master), but with perpendicular polarizationdirections. By this configuration, large data packets (e.g., first data308 and second data 312) can be split and simultaneously sent alongseparate pathways before recombination as downstream modulated datastream 172 (Ch1). Alternatively, first data 308 and second data 312 maycome from two (or more) separate unrelated sources. The orthogonal splitprevents data interference between the polarized signal components.However, one of ordinary skill in the art will appreciate that,according to the embodiment of FIG. 3, first unmodulated signal 168(Ch1) can also be polarized at 60 degrees, utilizing similar principlesof amplitude and phase, as well as wavelength division. Firstunmodulated signal 168 (Ch1) can alternatively be multiplexed accordingto a spiral or vortex polarization, or orbital angular momentum.Additionally, whereas the illustrated embodiment features polarizationmultiplexing, space division multiplexing and mode division multiplexingmay be also alternatively implemented.

According to this exemplary embodiment, master continuous wave signalfor Ch1, namely, first unmodulated signal 168, is received from opticalfrequency comb generator 114 and is split to be used, in the first part,as the LO for upstream receiver 132, and in the second part, tosynchronize two slave lasers (i.e., first laser diode 306 and secondlaser diode 310) by the respective x-polarization and y-polarizationlight portions such that both slave lasers oscillate according to thewavelength of the master laser (i.e., external laser 118). Data (i.e.,first data 308 and second data 312) is directly modulated onto the twoslave lasers, respectively. This injection locking technique thusfurther allows for frequency modulation (FM) noise spectrum control fromthe master laser to the slave laser, and is further able to realizesignificant improvements in FM noise/phase jitter suppression andemission linewidth reduction.

As described herein, utilization of optical injection with adual-polarization optical transmitter (i.e., downstream transmitter 300)by direct modulation may advantageously implement relatively lower-costlasers to perform the functions of conventional lasers that areconsiderably more costly. According to this configuration of adual-polarization optical transmitter by direct modulation ofsemiconductor laser together with coherent detection, the presentembodiments are particular useful for shortreach applications in termsof its lower cost and architectural compactness. Similar advantages maybe realized for long reach applications.

FIG. 4 is a schematic illustration depicting an alternative downstreamtransmitter 400 that can be utilized with fiber communication system100, depicted in FIG. 1. Downstream transmitter 400 is similar todownstream transmitter 200 (FIG. 2), except that downstream transmitter400 alternatively implements external modulation, as opposed to directmodulation, to modulate the Ch1 signal into downstream modulated datastream 172 (Ch1). Downstream transmitter 400 includes downstream opticalcirculator 136 (see FIG. 1, above) and a laser injected modulator 402.Downstream optical circulator 136 is in one-way direct communicationwith a separate external optical circulator 404 that may be containedwithin laser injected modulator 402 or separate. Laser injectedmodulator 402 further includes a laser diode 406, which receives the lowamplitude, narrow bandwidth, first unmodulated signal 168 (Ch1) andemits an excited, high amplitude, narrow bandwidth, optical signal 408back to external optical circulator 404. Laser injected modulator 402still further includes an external modulating element 410, whichreceives data 412 from an external data source 414, and adheres data 412with optical signal 408 to be unidirectionally received back bydownstream optical circulator 136 and output as downstream modulateddata stream 172 (Ch1).

In this exemplary embodiment, downstream transmitter 400 performs thesame general functions as downstream transmitter 126 (FIG. 1, describedabove), but uses external modulation as the injection locking mechanismto lock laser diode 406 to the wavelength of the master laser source(e.g., external laser 118). To implement external modulation, thisembodiment regulates optical signal flow through mostly unidirectionaloptical circulators (i.e., downstream optical circulator 136, externaloptical circulator 404). External modulating element 410 may optionallyinclude a demultiplexing filter (not shown) as an integral component, orseparately along the signal path of downstream modulated data stream 172(Ch1) prior to input by downstream receiver 150. In an exemplaryembodiment, external modulating element 410 is a monitor photodiode, andinjection locking is performed through a rear laser facet.

FIG. 5 is a schematic illustration depicting an alternative downstream500 transmitter that can be utilized with fiber communication system100, depicted in FIG. 1. Downstream transmitter 500 is similar todownstream transmitter 300 (FIG. 3), including the implementation ofdirect modulation and polarization division multiplexing, except thatdownstream transmitter 500 further implements quadrature amplitudemodulation (QAM) to modulate the Ch1 signal into downstream modulateddata stream 172 (Ch1). That is, further external modulating elements maybe utilized per polarization branch (FIG. 2, above) to generate QAMsignals.

Downstream transmitter 500 includes downstream optical circulator 136(see FIG. 1, above) in two-way communication with a laser injectedmodulator 502, which includes a PBS/PBC 504, which can be a singledevice or two separate devices. Additionally, all of the components oflaser injected modulator 502 may themselves be separate devices, oralternatively all contained within a single photonic chip. Laserinjected modulator 502 further includes a first laser diode 506configured to receive first data 508 from an external data source (notshown in FIG. 5), a second laser diode 510 configured to receive seconddata 512 from the same, or different, external data source, a thirdlaser diode 514 configured to receive third data 516 from thesame/different, external data source, and a fourth laser diode 518configured to receive fourth data 520 from the same/different externaldata source.

In operation, downstream transmitter 500 implements dual-polarizationfrom the splitter portion of PBS/PBC 504, which splits first unmodulatedsignal 168 (Ch1) into its x-polarization component (P1) andy-polarization component (P2). Each polarization component P1, P2 isthen input to first non-polarized optical splitter/combiner 522 andsecond non-polarized optical splitter/combiner 524, respectively. Firstand second optical splitters/combiners 522, 524 each then further splittheir respective polarization components P1, P2 into their I-signals526, 528, respectively, and also into their Q-signals 530, 532,respectively. Generated I-signals 526, 528 then directly excite laserdiodes 506, 514, respectively. Before directly communicating with laserdiodes 510, 518, respectively, generated Q-signals 530, 532 first passthrough first and second quadrature phase shift elements 534, 536,respectively, each of which shifts the Q-signal by 45 degrees in eachdirection, such that the respective Q-signal is offset by 90 degreesfrom its respective I-signal when recombined at splitters/combiners 522,524.

The resultant modulated Ch1 signal, with adhered data, is output fromdownstream optical circulator 136 of downstream transmitter 500 asdownstream modulated data stream 172 (Ch1), and as a polarized,multiplexed QAM signal. According to this exemplary embodiment,utilization of a photonic integrated circuit allows for directlymodulated polarization of a multiplexed coherent system, but utilizingsignificantly lower cost hardware configurations than are realized byconventional architectures. In an exemplary embodiment, laser diodes506, 510, 514, 516 are PAM-4 modulated laser diodes capable ofgenerating 16-QAM polarization multiplexed signals.

FIG. 6 is a schematic illustration depicting an exemplary upstreamtransmitter 600 that can be utilized with the fiber communication system100, depicted in FIG. 1. In the embodiment illustrated in FIG. 6,upstream transmitter 600 is similar to downstream transmitter 300 (FIG.3) in structure and function. Specifically, upstream transmitter 600includes upstream optical circulator 162 (see FIG. 1, above) in two-waycommunication with a laser injected modulator 602 (not separatelyillustrated in FIG. 6), which includes a PBS/PBC 604, which can be asingle device or separate devices. Laser injected modulator 602 furtherincludes a first laser diode 606 configured to receive first data 608from an external data source (not shown in FIG. 6), and a second laserdiode 610 configured to receive second data 612 from the same, ordifferent, external data source. Similar to the embodiments of FIGS.2-5, above, downstream transmitter 600 may also eliminate for upstreamoptical circulator 162 by the utilization of at least two separate fiberreceivers (not shown).

Upstream transmitter 600 is thus nearly identical to downstreamtransmitter 300 (FIG. 3), except that upstream transmitter 600 utilizessecond unmodulated signal 170 (Ch1′) as the end user seed source, inlaser injected modulator 602, to combine or adhere with data (e.g.,first data 608, second data 612) to generate upstream modulated datastream 176 (Ch1′) to carry upstream data signals to an upstream receiver(e.g., upstream receiver 132). In operation, first laser diode 606 andsecond laser diode 610 also function as slave lasers by injectionlocking to the master signal from external laser 118. That is, symmetricor asymmetric data for Ch1′ (e.g., first data 608, second data 612) ismodulated onto the two slave lasers (i.e., first laser diode 606 andsecond laser diode 610) with polarization multiplexing, much the same asthe process implemented with respect to downstream transmitter 300 (FIG.3) in optical hub 102.

In this example, upstream transmitter 600 is illustrated tosubstantially mimic the architecture of downstream transmitter 300 (FIG.3). Alternatively, upstream transmitter 600 could equivalently mimic thearchitecture of one or more of downstream transmitters 200 (FIG. 2), 400(FIG. 4), or 500 (FIG. 5) without departing from the scope of thepresent disclosure. Furthermore, upstream transmitter 600 can conform toany of the embodiments disclosed by FIGS. 2-5, irrespective of thespecific architecture of the particular downstream transmitter utilizedwithin optical hub 102. By utilization of high-quality, narrowbandwidth, low noise external laser source 118, the master/slave laserrelationship carries through the entirety of system 100, and theplurality of end users 106 that receive modulated/unmodulated signalpairs (which may be 32, 64, 128, or as many as 256 from a single fiberline pair, e.g., downstream fiber 108 and upstream fiber 110).

The significant cost savings according to the present embodiments arethus best realized when considering that as many as 512 downstreamtransmitters (e.g., downstream transmitter 126, FIG. 1) and upstreamtransmitters (e.g., upstream transmitter 160, FIG. 1) may be necessaryto fully implement all available chattel pairs from a single optical hub102. The present embodiments implement a significantly lower cost andless complex hardware architecture to utilize the benefits accruing fromimplementation of high-quality external laser 118, without having to addexpensive single longitudinal mode laser diodes, or other compensationhardware necessary to suppress adjacent longitudinal modes frominexpensive lasers or the noise components produced thereby.

FIG. 7 is a schematic illustration depicting an exemplary processingarchitecture which can be implemented for upstream receiver 132,downstream receiver 150, and fiber communication system 100, depicted inFIG. 1. The respective architectures of upstream receiver 132 anddownstream receiver 150 are similar with respect to form and function(described above with respect to FIG. 1), except that upstream receiver132 receives a first data stream pair 700 for Ch1, Ch1′, in reverse of asecond data stream pair 702, which is received by downstream receiver150. In other words, as described above, first data stream pair 700includes first unmodulated signal 168 (Ch1) as the LO and upstreammodulated data stream 176 (Ch1′) to carry data, whereas second datastream pair 702 includes unmodulated signal 170 (Ch1′) as the LO anddownstream modulated data stream 172 (Ch1) to carry data.

First and second data stream pairs 700, 702 the multiplexed phasesynchronized pairs modulated/unmodulated of optical signals that areconverted into analog electrical signals by ICR 140 and ICR 152,respectively. The respective analog signals are then converted intodigital domain by ADC 142 and ADC 154, for digital signal processing byDSP 144 and DSP 156. In an exemplary embodiment, digital signalprocessing may be performed by a CMOS ASIC employing very largequantities of gate arrays. A conventional CMOS ASIC, for example, canutilize as many as 70 million gates to process incoming digitized datastreams. In the conventional systems, modulated data streams for Ch1 andCh1′ are processed independently, which requires significant resourcesto estimate frequency offset, drift, and digital down conversioncompensation factors (e.g., e{circumflex over ( )}−jωt, where ωrepresents the frequency difference between first unmodulated signal 168and upstream modulated data stream 176, and ω is held constant forcoherent tone pair 166, as extended throughout system 100).

According to the exemplary embodiments disclosed herein, on the otherhand, the modulated and unmodulated signals from Ch1 and Ch1′ are phasesynchronized together such that the difference between w of the signalpair is always known, and phase synchronized to maintain a constantrelationship. In contrast, conventional systems are required toconstantly estimate the carrier phase to compensate for factors such asdraft which requires considerable processing resources, as discussedabove. According to the present embodiments though, since Ch1 and Ch1′are synchronized together as first and second data stream pairs 700,702, the offset w between the pairs 700, 702 need not be estimated,since it may be instead easily derived by a simplified subtractionprocess in DSP 144 and DSP 156 because the signal pairs will drifttogether by the same amount in a constant relationship. By thisadvantageous configuration and process, digital signal processing by aCMOS ASIC can be performed utilizing as few as one million gates,thereby greatly improving the processing speed of the respective DSP,and/or reducing the number of physical chips required to perform theprocessing (or similarly increasing the amount of separate processingthat may be performed by the same chip). At present, implementation ofthe embodiments described herein may improve downstream and upstreamdata transmission speeds by as much as 5000 times faster thanconventional systems.

FIG. 8 is a flow chart diagram of an exemplary downstream opticalnetwork process 800 that can be implemented with fiber communicationsystem 100, depicted in FIG. 1. Process 800 begins at step 802. In step802, coherent tone pairs 166 are generated and output by opticalfrequency comb generator 114, amplifier 122, and first hub opticaldemultiplexer 124. Similar to the discussion above, for simplificationpurposes, the following discussion addresses specific coherent tone pair166(1) for Ch1, Ch1′. Coherent tone pair 166 includes first unmodulatedsignal 168 (Ch1) and second unmodulated signal 170 (Ch1′). Once coherenttone pair 166 is generated, process 800 proceeds from step 802 to steps804 and 806, which may be performed together or simultaneously.

In step 804, first unmodulated signal 168 (Ch1) is input to an opticalsplitter, e.g., optical splitter 130, FIG. 1. In step 806, secondunmodulated signal 170 (Ch1′) is transmitted to a multiplexer, e.g., huboptical multiplexer 128, FIG. 1. Referring back to step 804, firstunmodulated signal 168 (Ch1) is split to function both as an LO forupstream detection, and as a seed for downstream data transmission. Forupstream detection, step 804 proceeds to step 808, where firstunmodulated signal 168 (Ch1) is received by an upstream receiver, i.e.,upstream receiver 132, FIG. 1. For downstream data transmission, step804 separately and simultaneously proceeds to step 810.

Step 810 is an optional step, where polarization division multiplexingis desired. In step 810, first unmodulated signal 168 (Ch1) is splitinto its x-component and y-component parts P1, P2, respectively (e.g.,by PBS/PBC 304, FIG. 3 or PBS/PBC 504, FIG. 5) for separate direct orexternal modulation. Where polarization division multiplexing is notutilized, process 800 skips step 810, and instead proceeds directly fromstep 804 to step 812. In step 812, first unmodulated signal 168 (Ch1),or its polarized components if optional step 810 is implemented, ismodulated by direct (e.g., FIGS. 2, 3, 5) or external (e.g., FIG. 4)modulation. Process 800 then proceeds from step 812 to step 814. Step814 is an optional step, which is implemented if optional step 810 isalso implemented for polarization division multiplexing. In step 814,the x-component and y-component parts P1, P2 are recombined (e.g., byPBS/PBC 304, FIG. 3 or PBS/PBC 504, FIG. 5) for output as downstreammodulated data stream 172 (Ch1). Where polarization divisionmultiplexing was not utilized, process 800 skips step 814, and insteadproceeds directly from step 812 to step 816.

In step 816, second unmodulated signal 170 (Ch1′) and downstreammodulated data stream 172 (Ch1) are optically multiplexed, i.e., by huboptical multiplexer 128, FIG. 1, as a phase synchronized data streampair (e.g., second data stream pair 702, FIG. 7). Process 800 thenproceeds from step 816 to step 818, where the phase synchronized datastream pair is transmitted over an optical fiber, i.e., downstream fiber108, FIG. 1. Process 800 then proceeds from step 818 to step 820, wherethe synchronized data stream pair is optically demultiplexed, e.g., bynode optical demultiplexer 174 in fiber node 104. Process 800 thenproceeds from step 820 to step 822, where both components of thedemultiplexed data stream pair (e.g., second unmodulated signal 170(Ch1′) and downstream modulated data stream 172 (Ch1)) are received by adownstream receiver (e.g., downstream receiver 150, FIG. 1) forheterodyne coherent detection.

Where an end user (e.g., end user 106) further includes upstreamtransmission capability, process 800 further includes optional steps 824and 826. In step 824, and prior to downstream reception in step 822,second unmodulated signal 170 (Ch1′) is optically split (e.g., by enduser optical splitter 158, FIG. 1), and additionally transmitted, instep 826, to an upstream transmitter of the end user (e.g., upstreamtransmitter 160, FIG. 1) as a seed signal for a modulator (e.g.,modulator 164, FIG. 1) for upstream data transmission, as explainedfurther below with respect to FIG. 9.

FIG. 9 is a flow chart diagram of an exemplary upstream optical networkprocess 900 that can be optionally implemented with fiber communicationsystem 100, depicted in FIG. 1. Process 900 begins at optional step 902.In step 902, where polarization division multiplexing is utilized in theupstream transmitter (e.g., upstream transmitter 160, FIG. 1), secondunmodulated signal 170 (Ch1′) (from step 826, FIG. 8) is split into itsx-component and y-component parts (e.g., by PBS/PBC 604, FIG. 6) forseparate direct or external modulation. Where polarization divisionmultiplexing is not utilized, step 902 is skipped, and process 900instead begins at step 904.

In step 904, second unmodulated signal 170 (Ch1′), or its polarizedcomponents if optional step 902 is implemented, is injection locked tothe master source laser (e.g., external laser 118, FIG. 1), as describedabove with respect to FIGS. 1 and 6. Step 904 then proceeds to step 906,where injection locked signal is modulated by direct or externalmodulation. Process 900 then proceeds from step 906 to step 908. Step908 is an optional step, which is implemented if optional step 902 isalso implemented for polarization division multiplexing. In step 908,the x-component and y-component parts of the excited Ch1′ signal arerecombined (e.g., by PBS/PBC 604, FIG. 6) for output as upstreammodulated data stream 176 (Ch1′). Where polarization divisionmultiplexing was not utilized, process 900 skips step 908, and insteadproceeds directly from step 906 to step 910.

In step 910, upstream modulated data stream 176 (Ch1′) is opticallymultiplexed, i.e., by node optical multiplexer 178, FIG. 1, with otherupstream data stream signals (not shown). Process 900 then proceeds fromstep 910 to step 912, where upstream modulated data stream 176 (Ch1′) istransmitted over an optical fiber, i.e., upstream fiber 110, FIG. 1.Process 900 then proceeds from step 912 to step 914, where upstreammodulated data stream 176 (Ch1′) is optically demultiplexed, e.g., bysecond hub optical demultiplexer 134, which separates the selected datastream from the other upstream data stream signals, for transmission toa particular upstream receiver tuned to receive the modulated datastream. Process 900 then proceeds from step 914 to step 916, where bothcomponents (e.g., first unmodulated signal 168 (Ch1), FIG. 8, andupstream modulated data stream 176 (Ch1′)) of the upstream data streampair, e.g., first data stream pair 700, FIG. 7, are received by anupstream receiver (e.g., upstream receiver and 32, FIG. 1) forheterodyne coherent detection.

As illustrated in the exemplary embodiment, a difference betweenupstream and downstream signal transmission is that an entiresynchronized modulated/unmodulated channel pair (e.g., second datastream pair 702, FIG. 7) can be transmitted in the downstream direction,whereas, in the upstream direction, only a data modulated signal (e.g.,upstream modulated data stream 176 (Ch1′)) to be transmitted over theupstream fiber connection, i.e., upstream fiber 110. An advantage of thepresent configuration is that the LO for upstream coherent detection(e.g., at upstream receiver 132, FIG. 1) comes directly from the splitsignal, i.e., first unmodulated signal 168 (Ch1) generated from opticalfrequency comb generator 114 within optical hub 102, after separation byfirst hub optical demultiplexer 124, as depicted in FIG. 1. Conventionalsystems typically require LO generation at each stage of the respectivesystem. According to the present disclosure, on the other hand,relatively inexpensive slave lasers can be implemented throughout thesystem architecture for modulation and polarization multiplexing in bothoptical hub 102 and end user 106 components, without requiring anadditional LO source at the end user.

According to the present disclosure, utilization of dual-polarizationoptical transmitters, and by direct modulation of semiconductor laserswith coherent detection, is particularly beneficial for not onlylonghaul applications, but also for shortreach applications to reducethe cost of electronic hardware, while also rendering the overallnetwork system architecture more compact. The present systems andmethods further solve the conventional problem of synchronizing twolaser sources over a long period of time. Utilization of the phasesynchronized data stream pairs and slave lasers herein allows continualsynchronization of the various laser sources throughout the systemduring its entire operation. These solutions can be implemented withincoherent DWDM-PON system architectures for access networks in acost-efficient manner.

Utilization of the high quality optical comb source at the front end ofthe system thus further allows a plurality of simultaneous narrowbandwidth wavelength channels to be generated with easily controlledspacing, and therefore also simplified tuning of the entire wavelengthcomb. This centralized comb light source in the optical hub providesmaster seeding sources and LO signals that can be reused throughout thesystem, and for both downstream and upstream transmission. Theimplementation of optical injection, as described herein, furtherimproves the performance of low-cost multi-longitudinal slave lasersources in terms of spectral bandwidth and noise properties. Accessnetworks according to the present systems and methods thus achieve moreefficient transmission of wavelengths through optical fibers, therebyincreasing the capacity of transmitted data, but at lower power,increased sensitivity, lower hardware cost, and a reduction indispersion, DSP compensation, and error correction.

Exemplary embodiments of fiber communication systems and methods aredescribed above in detail. The systems and methods of this disclosurethough, are not limited to only the specific embodiments describedherein, but rather, the components and/or steps of their implementationmay be utilized independently and separately from other componentsand/or steps described herein. Additionally, the exemplary embodimentscan be implemented and utilized in connection with other access networksutilizing fiber and coaxial transmission at the end user stage.

This written description uses examples to disclose the embodiments,including the best mode, and also to enable any person skilled in theart to practice the embodiments, including making and using any devicesor systems and performing any incorporated methods. The patentable scopeof the disclosure is defined by the claims, and may include otherexamples that occur to those skilled in the art. Such other examples areintended to be within the scope of the claims if they have structuralelements that do not differ from the literal language of the claims, orif they include equivalent structural elements with insubstantialdifferences from the literal language of the claims.

Although specific features of various embodiments of the disclosure maybe shown in some drawings and not in others, this is for convenienceonly. In accordance with the principles of the disclosure, a particularfeature shown in a drawing may be referenced and/or claimed incombination with features of the other drawings.

Some embodiments involve the use of one or more electronic or computingdevices. Such devices typically include a processor or controller, suchas a general purpose central processing unit (CPU), a graphicsprocessing unit (GPU), a microcontroller, a reduced instruction setcomputer (RISC) processor, an application specific integrated circuit(ASIC), a programmable logic circuit (PLC), a field programmable gatearray (FPGA), a DSP device, and/or any other circuit or processorcapable of executing the functions described herein. The processesdescribed herein may be encoded as executable instructions embodied in acomputer readable medium, including, without limitation, a storagedevice and/or a memory device. Such instructions, when executed by aprocessor, cause the processor to perform at least a portion of themethods described herein. The above examples are exemplary only, andthus are not intended to limit in any way the definition and/or meaningof the term “processor.”

This written description uses examples to disclose the embodiments,including the best mode, and also to enable any person skilled in theart to practice the embodiments, including making and using any devicesor systems and performing any incorporated methods. The patentable scopeof the disclosure is defined by the claims, and may include otherexamples that occur to those skilled in the art. Such other examples areintended to be within the scope of the claims if they have structuralelements that do not differ from the literal language of the claims, orif they include equivalent structural elements with insubstantialdifferences from the literal language of the claims.

What is claimed is:
 1. A downstream transceiver for an opticalcommunication network, comprising: an input portion configured toreceive an input laser signal from a remote primary laser source of theoptical communication network, the input laser signal including acoherent tone pair having (i) a first coherent tone containing adownstream modulated signal, and (ii) a second coherent tone containinga downstream unmodulated signal; and a laser injected modulatorincluding a first secondary laser having a resonator frequency injectionlocked to a primary frequency of a single longitudinal mode of theremote primary laser source, wherein a frequency of the second coherenttone is held constant to a frequency of the first coherent tone, andwherein the laser injected modulator is configured to receive thedownstream unmodulated signal on the second coherent tone, and output anupstream modulated laser signal.
 2. The downstream transceiver of claim1, wherein the laser injected modulator is configured to implementdirect modulation.
 3. The downstream transceiver of claim 1, wherein thelaser injected modulator is configured to implement external modulation.4. The downstream transceiver of claim 1, wherein the first secondarylaser comprises at least one of an LED, a Fabry Perot laser diode, and avertical-cavity surface-emitting laser.
 5. The downstream transceiver ofclaim 1, further comprising a first optical circulator in communicationwith the laser injected modulator and the unmodulated signal on thesecond coherent tone.
 6. The downstream transceiver of claim 5, whereinthe laser injected modulator is configured to implement one ofpolarization division multiplexing, space division multiplexing, andmode division multiplexing.
 7. The downstream transceiver of claim 6,wherein the laser injected modulator is configured to multiplex theinput laser signal at one of an orthogonal polarization, a 60 degreepolarization, a 90 degree polarization, a spiral polarization, acircular polarization, a vortex polarization, or an orbital angularmomentum.
 8. The downstream transceiver of claim 5, further comprising apolarization beam combiner (PBC) disposed between the first opticalcirculator and the first secondary laser.
 9. The downstream transceiverof claim 8, further comprising a second secondary laser, wherein thefirst secondary laser is configured to modulate an x-component of theupstream laser modulated data stream, and wherein the second secondarylaser is configured to modulate a y-component of the upstream modulatedlaser signal.
 10. The downstream transceiver of claim 9, wherein thelaser injected modulator is configured to implement quadrature amplitudemodulation.
 11. The downstream transceiver of claim 10, furthercomprising: a first optical splitter disposed between the PBC and thefirst secondary laser; and a second optical splitter disposed betweenthe PBC and the second secondary laser, wherein the first secondarylaser comprises a first sub-laser and a second sub-laser, wherein thesecond secondary laser comprises a third sub-laser and a fourthsub-laser, wherein the first sub-laser is configured to receive anI-signal of the x-component, wherein the second sub-laser is configuredto receive a Q-signal of the x-component, wherein the third sub-laser isconfigured to receive an I-signal of the y-component, wherein the fourthsub-laser is configured to receive a Q-signal of the y-component. 12.The downstream transceiver of claim 11, further comprising: a firstphase shift element disposed between the first optical splitter and thesecond sub-laser; and a second phase shift element disposed between thesecond optical splitter and the fourth sub-laser.
 13. The downstreamtransceiver of claim 5, further comprising: a downstream receiverconfigured to process the downstream modulated data stream on the firstcoherent tone; and a second optical circulator in one-way communicationwith the first optical circulator and in two-way communication with thedownstream receiver.
 14. The downstream transceiver of claim 13, whereinthe downstream receiver is a coherent receiver and is further configuredto receive the unmodulated signal on the second coherent tone as a localoscillator source.
 15. The downstream transceiver of claim 13, whereinthe downstream receiver is further configured to perform heterodynedetection.
 16. The downstream transceiver of claim 13, wherein thedownstream receiver comprises one or more of an integrated coherentreceiver, an analog to digital converter, and a digital signalprocessor.
 17. The downstream transceiver of claim 13, wherein the inputlaser signal further includes a plurality of downstream laser signalsfrom the remote primary laser source, and wherein the downstreamreceiver is further configured to multiplex the plurality of downstreamlaser signals into a single received coherent signal.
 18. The downstreamtransceiver of claim 1, comprising at least one of a downstreamtermination unit, a customer device, a customer premises equipment, amodem, an optical network unit (ONU), a data center, and a virtualizedcable modem termination system (vCMTS).
 19. The downstream transceiverof claim 1, wherein the first and second coherent tones are a phasesynchronized coherent tone pair, and wherein the first and secondcoherent tones are substantially confined to a narrow bandwidth of thesingle longitudinal mode of the remote primary laser source.
 20. Thedownstream transceiver of claim 1, wherein the first secondary laser isconfigured to receive data from a first external data source to adhereinto the output upstream modulated laser signal.